Such top drives provide for, inter alia, the rotary drive and the lowering and raising of the drill string. For this purpose, the drill string is coupled in a rotationally fixed manner with the main spindle, usually via a screw connection. The top drive is guided in a mast with the aid of a guide carriage during operation. Reaction moments are introduced into the mast during operation via the guide carriage.
Lowering and raising of the top drive, and thus the drill string, is usually provided by a pulley arrangement which is held at the upper end of the mast and connected, for example, via a mounting bracket, with the top drive.
For rotary actuation of the main spindle, the gear unit is connected with at least one electrically or hydraulically driven rotary motor.
Due to the fixed connection of the drill string with the main spindle, high demands are made on the bearing thereof in the gear housing of the gear unit of the top drive. An axial bearing must in particular be provided for the main spindle, the axial bearing being suitable to introduce the mass which is present in a drill string with a working head into the housing. The axial bearing must also permanently withstand the dynamic loads that occur during a drilling operation.
The axial bearing of the main spindle comprises an axial main bearing in a top drive of known construction. It is formed as a conical roller bearing. Its lower main bearing part abuts the housing. The main spindle is supported with a radial flange on its upper main bearing part.
In order to achieve a long service life of the axial bearing, play between the bearing rollers and the upper and lower bearing parts is to be avoided. A spring unit is provided for this purpose in known top drives, the spring unit pressing the upper auxiliary bearing part onto the rolling elements.
It has been shown, however, that in spite of the preloading which acts in this way, the axial main bearing may be subject to unexpectedly high wear and tear, in particular when drilling under pressure. The spring assemblies are hereby pressed together. Bearing play is present in the lower bearing. It is thus not provided that the rollers of the main bearing roll evenly.